The invention relates to a pulsed light source for removing biological tissue and particularly to controlling the light source for removing tissue or warming the irradiated area.
It is known from many medical applications that tissue can be removed or cut with the aid of adequately intensive light, in particular laser radiation. Removal is accompanied by heating of the surrounding tissue. The extent of this heating is determined in particular by the wavelength of the radiation used, or the coefficient of absorption of the tissue dependent on the latter, and by the irradiation intensity. In the case of high absorption in the tissue and low irradiation intensity, as is the case for example with a CO.sub.2 continuous-wave laser, tissue is pyrolytically vaporized with a relatively great thermal side effect. In a typical case, the crater or cut formed in soft tissue is surrounded by a carbonization layer, a zone broken up by vacuoles, a coagulation zone and a reversibly thermally damaged region. The coagulation of the tissue produced by the heating, and the accompanying hemostasis is of practical advantage in many cases, because it makes possible cuts which do not bleed. On the other hand, for applications in which as little damage as possible to the remaining tissue and good healing of the wound are important, great thermal effects are disadvantageous. Carbonization of the tissue surface, as occurs when cutting with continuous-wave lasers, is likewise unfavorable. It has already been attempted in the case of such lasers to reduce the thermal damage by increasing the irradiation intensity while at the same time shortening the time period in which it acts.
On the other hand, research in recent years has shown that, with pulsed light sources of high power and a wavelength in the ultraviolet or infrared ranges, for example TEA-CO.sub.2, Er:YAG, Er:YSGG or excimer lasers, hard or soft tissue can be removed without carbonization and with only little thermal damage by a very effective thermomechanical ablation process. For instance, in the case of soft tissue, the marginal edge which coagulated after use of the free-running Er:YAG laser in vivo is only about 30-40 .mu.m. This is of particular interest for the treatment of superficial skin lesions or for cosmetic surgery, because damage of the tissue beyond that which is removed is largely avoided. If, however, the capillary layer of the tissue is reached, the removal is stopped by emerging blood.
In all the surgical applications of light sources used thus far, the removal properties and the thermal side effects are coupled in as much as precise removal with high removal efficiency is always accompanied by a small thermal side effect, and vice versa. A known possible way of achieving different thermal side effects is that of combining a plurality of light sources of different wavelengths in one device. However, the parallel operation of the two light sources required for simultaneous cutting and coagulating requires high expenditure on apparatus.
DE 39 34 646 A1 discloses a method and an apparatus of the type stated at the beginning in which a specifically directed vaporization without partial decomposition or burning is to be achieved by the luminous effect occurring in the pyrolysis process being used as a control signal for the control unit. By this means, the control unit is controlled in such a way that either the laser power, the clock ratio or the pulse energy are changed.
Furthermore, DE 32 33 671 A1 discloses a laser apparatus having a memory means for storing a multiplicity of data records, which specify parameters for the operating conditions for a particular laser radiation. Details on the individual parameters, and consequently an optimum removal of tissue, are not specified here however.